Fix lesson order being changed when saving lesson

[hansschuijff]

Fixes #3921

Changes proposed in this Pull Request

• Skip Sensei_Course_Structure::sort_structure sorting when $order is empty ( or [0] ).

In analysing the problem that on single lesson save, the entire lesson order can be changed, I found that the php usort function changes the lesson order when all lessons return a 0 value.

Note: The entry with value 0 is the result of the way $order is filled from the (empty) $order_string parameter in Sensei_Admin->save_lesson_order().

$order = array_map( 'absint', explode( ',', $order_string ) );

In those cases the lesson order will already be sorted correctly and the intention of the callback function used in this method is to leave the structure unaffected.

This PR just makes the usort-call conditional so it skips sorting when there is nothing to sort ( e.g. $order equals [0] ).

Testing instructions

• I don't know exactly why and when usort fails, but in our case we had 47 lessons and no modules in the course. So all lessons where no module lessons.

New/Updated Hooks

Deprecated Code

Screenshot / Video

[yscik]

Thanks for the PR! Looks to be working well for all cases.

• New lessons added to the end of a course
• Ordering works everywhere as expected